The present invention relates to hue control for color television receivers wherein the hue setting is automatically obtained by use of a vertical interval reference (VIR) signal received as part of the video wave.
The present invention is an improvement over the invention set forth in copending patent application Ser. No. 500,082, VIR Chroma System, filed Aug. 23, 1974 by Harry T. Freestone, now U.S. Pat. No. 3,950,780, and assigned to the assignee of the present application. The Freestone application relates in part to a hue control circuit for controlling the hue setting of a color television receiver in response to a VIR signal. In the Freestone application a closed loop control circuit is described for continually updating the phase setting of the color difference detector circuit by comparison of the R-Y output of the color difference detector during the chrominance reference portion of the VIR signal with a known zero chrominance output to insure accuracy of the hue setting of the receiver under the control of the VIR signal. The closed loop hue control circuit insures that the hue setting is always correct regardless of deviations in the phase of the burst signal as transmitted and regardless of drift due to aging of the receiver circuitry for processing the chrominance information. The present invention envisions the utilization of such a closed loop hue control circuit as described in the Freestone application and also allows for viewer preference of the hue setting.
The natural inclination in order to permit viewer preference control of the hue setting of the receiver is to add an offset voltage to the automatic hue control loop so that a fixed voltage is simply added to the hue setting determined under the control of the VIR signal. Unfortunately, such interjection of an offset voltage disturbs the null condition of the feedback loop established when the R-Y output is set to be equal to a known zero chrominance output. The null or zero condition established by the feedback control circuit insures that the hue setting is independent of the saturation setting, so that hue is unaffected by shifts in chroma level. If hue is allowed to become dependent upon the chroma level, expensive precision components are needed to maintain the proper relationship and insure consistent tracking between hue and saturation.
It is accordingly an object of the present invention to add viewer hue preference capability to a color television receiver wherein the hue setting is VIR controlled.
Another object of the present invention is to add viewer hue preference capability to such a color television receiver without resort to precision components.
Another object is to add viewer hue preference while maintaining independence between hue and saturation levels in the receiver.
These and other objects are realized by the present invention by the utilization of a gated adder circuit to selectively combine a viewer hue preference voltage with the VIR hue control voltage. The gated adder circuit is responsive to a line 19 timing pulse to permit hue to be established exclusively under control of the VIR signal during line 19. This allows hue to remain independent of chroma levels and saturation setting. At times other than line 19 the gated adder combines the viewer hue preference voltage with the VIR control voltage to effect fixed offset of the hue setting.